1. Field of the Invention
This invention deals with photography and, more particularly, with products and processes wherein photosensitive silver halide emulsions are spectrally (optically) sensitized to the green region of the visible spectrum.
2. Description of the Prior Art
Those skilled in the photograhic art are aware that the photoresponse of an ordinary silver halide emulsion is generally limited to the blue and violet regions of the visible spectrum. However, it has been found that the addition of certain cyanine and related dyes to a silver halide emulsion extends the emulsion's photoresponse approximately to the spectral regions absorbed by said dyes, when said dyes are adsorbed onto the surface of the silver halide crystal. The cyanine dyes are those conforming to an amidinium ion system in which both nitrogen atoms are included in separate heterocyclic ring systems and in which the conjugated chain joining the nitrogen atoms passes through a part of each heterocyclic ring.
Generally speaking, unsymmetrical carbocyanine dyes wherein one of the heterocyclic ring systems is of the benzimidazole series and the other is of the benzothiazole series have previously been described in the art as being useful sensitizers for photographic silver halide emulsions. In the patent literature, such unsymmetrical dyes have been described in, for example, U.S. Pat. Nos. 2,289,300; 2,323,187; 2,778,823; 2,918,369; 3,264,110; and 3,706,570; and British Pat. Nos. 505,979; 955,964; 1,045,777; 1,132,528; and 1,328,239, and Defensive Publication No. T87,016 of application Ser. No. 884,747, filed Dec. 12, 1969 and published May 26, 1970 in 874 O.G. 1022.
However, many of these prior art benzimidazolothiacarbocyanine dyes exhibit too much red light absorption to be completely satisfactory as green sensitizers for silver halide emulsions, and others provide green sensitization which in many instances falls short of preferred levels for certain recording applications. Still other dyes may exhibit poor stability, fogging, antisensitization, or incompatability with other components in preferred photographic systems.
The present invention is directed to improved green sensitization of silver halide utilizing specific benzimidazolo-thiacarbocyanine dyes detailed hereinafter, alone or in combination with other cyanine dyes, thereby avoiding the deficiencies of prior art cyanine dyes with similar structure.